1. Field of the Invention
The present invention relates to a spark plug for igniting an internal combustion engine.
2. Description of the Related Art
The metallic shell of a spark plug is fixedly attached to an insulator by means of crimping. Specifically, the insulator is inserted into the metallic shell formed into a tubular shape, and then by use of dies a compressive load is applied to the peripheral edge of a rear end portion (a portion to be crimped) of the metallic shell. By this procedure, the portion to be crimped is curved toward a flange-like protrusion formed on the outer circumferential surface of the insulator to thereby become a crimped portion, whereby the insulator is fixed in place. The metallic shell is generally formed from a steel material such as carbon steel.
A method for firmly joining the insulator 2 to the metallic shell 1 by means of the crimped portion 1d is specifically carried out in the following manner. As shown in FIG. 2(a), when a portion-to-be-crimped 1d′ is axially compressed by means of crimping die 111, the portion-to-be-crimped 1d′ is plastically deformed radially inward. A thread packing 61, for example, is disposed between the portion-to-be deformed 1d′ and a flange-like protrusion 2e. When compressive deformation of the portion-to-be-crimped 1d′ increases, a load begins to be imposed on the thread packing 61 and the flange-like protrusion 2e (hereinafter, these are generically and collectively called a “portion to be compressed”). While the portion to be compressed undergoes compressive deformation, plastic deformation of the portion-to-be-crimped 1d′ proceeds further. Then, as shown in FIG. 2(b) which is a step following the step shown in FIG. 2(a), when a final value for a compression stroke for crimping is reached, unloading is performed to thereby complete the crimping process (the portion-to-be-crimped 1d′ becomes a crimped portion 1d). The unloading induces some springback of the crimped portion 1d. However, since the crimped portion 1d is plastically deformed, the crimped portion 1d retains the compressed portion in an elastically deformed condition, thereby inducing a fastening force for firmly joining the insulator 2 to the metallic shell 1. In some cases, the thread packing 61 may not be provided.
The above-mentioned crimping process is performed, for example, in the following manner. Crimping is performed while electricity is supplied to the metallic shell via the die to thereby heat to, for example, 700° C. or higher a thin-walled portion 1h formed between two protrusions (a tool engagement portion 1e and a flange-like gas seal portion 1g) so as to reduce deformation resistance; i.e., crimping is performed while deformation resistance is reduced. This crimping process is called hot crimping. Hot crimping can utilize the thermal expansion difference between the metallic shell 1 and the insulator 2 for crimping, whereby a highly gastight crimped structure can be readily obtained.
3. Problems Solved by the Invention
Along with a recent tendency of an engine toward complex arrangement around heads and an increase in valve diameter, spark plugs show a marked tendency towards a decrease in diameter and increase in length. However, decreasing the diameter of a spark plug requires employing a metallic shell having a small diameter and a thin wall. As is apparent from the above-described principle, a force for fastening the insulator against the metallic shell is induced by reaction from the crimped portion 1d. Since a reduction in the diameter and wall thickness of the metallic shell is accompanied by a reduction in the cross-sectional area of the crimped portion 1d, bringing stress arising on the cross section of the crimped portion 1d to the same level as a conventional one requires a reduction in compression stroke for crimping. Thus, total fastening force decreases by an extent corresponding to the reduction in the cross-sectional area. As a result, gas tightness established between the metallic shell and the insulator is deteriorated. Particularly, when harsh vibrations act on a spark plug as in high-speed, high-load driving, crimping of the spark plug may be loosened, and thus gastightness is more likely to deteriorate.
By contrast, an attempt to maintain the total fastening force at the same level as a conventional one involves an increase in stress by an extent corresponding to a decrease in the cross-sectional area of the crimped portion 1d; as a result, the strength of the crimped portion 1d fails to endure the stress, thereby leading to a failure to maintain gastightness. In hot crimping, the thin-walled portion 1h rises in temperature as a result of supply of electricity thereto and is plastically deformed. Therefore, a reaction force stemming from thermal expansion difference is also imposed on the thin-walled portion 1h. Since electricity-effected temperature rise varies widely among metallic shells, a reaction force stemming from thermal expansion difference also varies; as a result, lack of strength arises in the crimped portion 1d, and particularly impaired gastightness is likely to arise.